1. Field of the invention
This invention relates to a silicon nitride circuit substrate and a semiconductor device in which a semiconductor element(s) is mounted on said substrate, particularly to a circuit substrate having excellent mechanical strength, heat cycle resistance and radiation characteristic.
2. Prior art
An electronic circuit mainly comprises an element such as IC, a substrate, a wiring and the like. In recent years, an electronic circuit has been speeded up, downsized and rendered large output possible, and the calorific value of an element has been a large value which cannot be ignored. In order to cope with such a situation, as a substitute for a conventional ceramic substrate, there have been developed, for example, aluminum nitride (AlN) having high thermal conductivity and a glass+ceramic substrate having a low dielectric constant.
Here, a substrate comprising aluminum nitride has excellent radiation property, but the substrate has a relatively high dielectric constant, and sintering temperature is high. Therefore, a conductive component is limited to a metal having a high melting point and relatively low conductivity such as W and Mo, and it has been desired to lower the dielectric constant of the substrate and the resistance of a wiring material.
Further, the thermal expansion coefficient is about 4.5.times.10.sup.-6 /.degree. C. which is smaller than that of alumina ceramics which is a conventional circuit substrate, so that at the time of metallization, breaking of ceramics is caused by mismatching of thermal expansion coefficients, whereby there is a problem of lowering reliability of a device using the substrate. On the other hand, a substrate comprising glass and ceramics has a low dielectric constant, and sintering temperature is low, so that it has a characteristic that a metal having a low melting point and high conductivity such as Au, Ag and Cu can be used as a wiring material. However, the substrate has a drawback that thermal conductivity and mechanical strength are low.
Further, as a novel circuit substrate, silicon nitride ceramics has been investigated in recent years. Silicon nitride has excellent mechanical characteristics such as flexural strength and toughness. Particularly, even at a high temperature of 1,000.degree. C. or higher, lowering of strength is small, and corrosion resistance is excellent, so that silicon nitride has been highlighted as a high temperature and high strength material such as a member for a gas turbine and an engine member. In the prior art, silicon nitride has been considered to be unsuitable for a circuit substrate because of low thermal conductivity thereof. However, in recent years, as disclosed in, for example, Japanese Provisional Patent Publication No. 135771/1994, improvement of thermal conductivity of silicon nitride ceramics has progressed, and silicon nitride ceramics having a thermal conductivity of about 120 W/mK has been obtained.